MPLS Working Group S. Bryant
Internet-Draft Huawei
Intended status: Informational C. Pignataro
Expires: June 17, 2018 Cisco Systems
M. Chen
Z. Li
Huawei
G. Mirsky
ZTE Corp.
December 14, 2017
MPLS Flow Identification Considerations
draft-ietf-mpls-flow-ident-06
Abstract
This document discusses the aspects that need to be be considered
when developing a solution for MPLS flow identification. The key
application that needs this is in-band performance monitoring of MPLS
flows when MPLS is used to encapsulate user data packets.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Loss Measurement Considerations . . . . . . . . . . . . . . . 3
4. Delay Measurement Considerations . . . . . . . . . . . . . . 4
5. Units of identification . . . . . . . . . . . . . . . . . . . 4
6. Types of LSP . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Network Scope . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 7
9. Dataplane . . . . . . . . . . . . . . . . . . . . . . . . . . 7
10. Control Plane . . . . . . . . . . . . . . . . . . . . . . . . 9
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9
12. Security Considerations . . . . . . . . . . . . . . . . . . . 9
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
15.1. Normative References . . . . . . . . . . . . . . . . . . 10
15.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
This document discusses the aspects that need to be considered when
developing a solution for MPLS flow identification.The key
application that needs this is in-band performance monitoring of MPLS
flows when MPL is used for the encapsulation of user data packets.
There is a need to identify flows in MPLS networks for various
applications such as determining packet loss and packet delay
measurement. A method of loss and delay measurement in MPLS networks
was defined in [RFC6374]. When used to measure packet loss [RFC6374]
depends on the use of injected Operations, Administration, and
Maintenance (OAM) packets to designate the beginning and the end of
the packet group over which packet loss is being measured. Where the
misordering of packets from one group relative to the following
group, or misordering of one of the packets being counted relative to
the [RFC6374] packet occurs, then an error will occur in the packet
loss measurement.
In addition, [RFC6374] did not support different granularities of
flow or address a number of multi-point cases in which two or more
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ingress Label Switching Routers (LSRs) could send packets to one or
more destinations.
Improvements in link and transmission technologies have made it more
difficult to assess packet loss using active performance measurement
methods with synthetic traffic, due to the very low loss rate in
normal operation. That, together with more demanding service level
requirements, means that network operators now need to be able to
measure the loss of the actual user data traffic by using passive
performance measurement methods. Any technique deployed needs to be
transparent to the end user, and it needs to be assumed that they
will not take any active part in the measurement process. Indeed it
is important that any flow identification technique be invisible to
them and that no remnant of the identification of measurement process
leaked into their network.
Additionally where there are multiple traffic sources, such as in
multi-point to point and multi-point to multi-point network
environments there needs to be a method whereby the sink can
distinguish between packets from the various sources, that is to say,
that a multi-point to multi-point measurement model needs to be
developed.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119].
3. Loss Measurement Considerations
Modern networks, if not oversubscribed, potentially drop relatively
few packets, thus packet loss measurement is highly sensitive to the
common demarcation of the exact set of packets to be measured for
loss. Without some form of coloring or batch marking such as that
proposed in [I-D.ietf-ippm-alt-mark] it may not be possible to
achieve the required accuracy in the loss measurement of customer
data traffic. Thus where accurate measurement of packet loss is
required, it may be economically advantageous, or even a technical
requirement, to include some form of marking in the packets to assign
each packet to a particular counter for loss measurement purposes.
Where this level of accuracy is required and the traffic between a
source-destination pair is subject to Equal-Cost Multipath (ECMP) a
demarcation mechanism is needed to group the packets into batches.
Once a batch is correlated at both ingress and egress, the packet
accounting mechanism is then able to operate on the batch of packets
which can be accounted for at both the packet ingress and the packet
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egress. Errors in the accounting are particularly acute in Label
Switched Paths (LSPs) subjected to ECMP because the network transit
time will be different for the various ECMP paths since:
1. The packets may traverse different sets of LSRs.
2. The packets may depart from different interfaces on different
line cards on LSRs.
3. The packets may arrive at different interfaces on different line
cards on LSRs.
A consideration with this solution on modifying the identity label
(the MPLS label ordinarily used to identify the LSP, Virtual Private
Network, Pseudowire etc) to indicate the batch is the impact that
this has on the path chosen by the ECMP mechanism. When the member
of the ECMP path set is chosen by deep packet inspection a change of
batch represented by a change of identity label will have no impact
on the ECMP path. Where the path member is chosen by reference to an
entropy label [RFC6790] then changing the batch identifier will not
result in a change to the chosen ECMP path. ECMP is so pervasive in
multi-point to (multi-) point networks that some method of avoiding
accounting errors introduced by ECMP needs to be supported.
4. Delay Measurement Considerations
Most of the existing delay measurement methods are active measurement
that depend on the extra injected test packet to evaluate the delay
of a path. With the active measurement method, the rate, numbers and
interval between the injected packets may affect the accuracy of the
results. Also, for injected test packets, these may not be co-routed
with the data traffic due to ECMP, or various link aggregation
technologies all of which distribute flows across a number of paths
at the network, or data-link and hence at the physical layer.
Thus there exists a requirement to measure the delay of the real
traffic.
For combined loss-delay measurements, both the loss and the delay
considerations apply.
5. Units of identification
The most basic unit of identification is the identity of the node
that processed the packet on its entry to the MPLS network. However,
the required unit of identification may vary depending on the use
case for accounting, performance measurement or other types of packet
observations. In particular note that there may be a need to impose
identify at several different layers of the MPLS label stack.
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This document considers following units of identifications:
o Per source LSR - everything from one source is aggregated.
o Per group of LSPs chosen by an ingress LSR - an ingress LSP
aggregates group of LSPs (ex: all LSPs of a tunnel).
o Per LSP - the basic form.
o Per flow [RFC6790] within an LSP - fine grained method.
Note that a fine grained identity resolution is needed when there is
a need to perform these operations on a flow not readily identified
by some other element in the label stack. Such fine grained
resolution may be possible by deep packet inspection, but this may
not always be possible, or it may be desired to minimize processing
costs by doing this only in entry to the network, and adding a
suitable identifier to the packet for reference by other network
elements. An example of such a fine grained case might be traffic
from a specific application, or from a specific application from a
specific source, particularly if matters related to service level
agreement or application performance were being investigated.
We can thus characterize the identification requirement in the
following broad terms:
o There needs to be some way for an egress LSR to identify the
ingress LSR with an appropriate degree of scope. This concept is
discussed further in Section 7.
o There needs to be a way to identify a specific LSP at the egress
node. This allows for the case of instrumenting multiple LSPs
operate between the same pair of nodes. In such cases the
identity of the ingress LSR is insufficient.
o In order to conserve resources such as labels, counters and/or
compute cycles it may be desirable to identify an LSP group so
that a operation can be performed on the group as an aggregate.
o There needs to be a way to identify a flow within an LSP. This is
necessary when investigating a specific flow that has been
aggregated into an LSP.
The unit of identification and the method of determining which
packets constitute a flow will be application or use-case specific
and is out of scope of this document.
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6. Types of LSP
We need to consider a number of types of LSP. The two simplest types
to monitor are point to point LSPs and point to multi-point LSPs.
The ingress LSR for a point to point LSP, such as those created using
the Resource Reservation Protocol - Traffic Engineering (RSVP-TE)
[RFC5420] Signaling protocol, or those that conform to the MPLS
Transport Profile (MPLS-TP) [RFC5654] may be identified by inspection
of the top label in the stack, since at any provider-edge (PE) or
provider (P) router on the path this is unique to the ingress-egress
pair at every hop at a given layer in the LSP hierarchy. Provided
that penultimate hop popping is disabled, the identity of the ingress
LSR of a point to point LSP is available at the egress LSR and thus
determining the identity of the ingress LSR must be regarded as a
solved problem. Note however that the identity of a flow cannot to
be determined without further information being carried in the
packet, or gleaned from some aspect of the packet payload.
In the case of a point to multi-point LSP, and in the absence of
Penultimate Hop Popping (PHP) the identity of the ingress LSR may
also be inferred from the top label. However, it may not possible to
adequately identify the flow from the top label alone, and thus
further information may need to be carried in the packet, or gleaned
from some aspect of the packet payload. In designing any solution it
is desirable that a common flow identity solution be used for both
point to point and point to multi-point LSP types. Similarly it is
desirable that a common method of LSP group identification be used.
In the above cases, a context label [RFC5331] needs to be used to
provide the required identity information. This is widely supported
MPLS feature.
A more interesting case is the case of a multi-point to point LSP.
In this case the same label is normally used by multiple ingress or
upstream LSRs and hence source identification is not possible by
inspection of the top label by the egress LSRs. It is therefore
necessary for a packet to be able to explicitly convey any of the
identity types described in Section 5.
Similarly, in the case of a multi-point to multi-point LSP the same
label is normally used by multiple ingress or upstream LSRs and hence
source identification is not possible by inspection of the top label
by egress LSRs. The various types of identity described in Section 5
are again needed. Note however, that the scope of the identity may
be constrained to be unique within the set of multi-point to multi-
point LSPs terminating on any common node.
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7. Network Scope
The scope of identification can be constrained to the set of flows
that are uniquely identifiable at an ingress LSR, or some aggregation
thereof. There is no need of an ingress LSR seeking assistance from
outside the MPLS protocol domain.
In any solution that constrains itself to carrying the required
identity in the MPLS label stack rather than in some different
associated data structure, constraints on the choice of label and
label stack size imply that the scope of identity resides within that
MPLS domain. For similar reasons the identity scope of a component
of an LSP is constrained to the scope of that LSP.
8. Backwards Compatibility
In any network it is unlikely that all LSRs will have the same
capability to support the methods of identification discussed in this
document. It is therefore an important constraint on any flow
identity solution that it is backwards compatible with deployed MPLS
equipment to the extent that deploying the new feature will not
disable anything that currently works on a legacy equipment.
This is particularly the case when the deployment is incremental or
the feature is not required for all LSRs or all LSPs. Thus, the flow
identification design MUST support the co-existence of both LSRs that
can, and cannot, identify the traffic components described in
Section 5. In addition the identification of the traffic components
described in Section 5 MUST be an optional feature that is disabled
by default. As a design simplification, a solution MAY require that
all egress LSRs of a point to multi-point or a multi- point to multi-
point LSP support the identification type in use so that a single
packet can be correctly processed by all egress devices. The
corollary of this last point is that either all egress LSRs are
enabled to support the required identity type, or none of them are.
9. Dataplane
There is a huge installed base of MPLS equipment, typically this type
of equipment remains in service for an extended period of time, and
in many cases hardware constraints mean that it is not possible to
upgrade its dataplane functionality. Changes to the MPLS data plane
are therefore expensive to implement, add complexity to the network,
and may significantly impact the deployability of a solution that
requires such changes. For these reasons, MPLS users have set a very
high bar to changes to the MPLS data plane, and only a very small
number have been adopted. Hence, it is important that the method of
identification must minimize changes to the MPLS data plane. Ideally
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method(s) of identification that require no changes to the MPLS data
plane should be given preferential consideration. If a method of
identification makes a change to the data plane is chosen it will
need to have a significant advantage over any method that makes no
change, and the advantage of the approach will need to be carefully
evaluated and documented. If a change is necessary to the MPLS data
plane proves necessary, it should be (a) be as small a change as
possible and (b) be a general purpose method so as to maximize its
use for future applications. It is imperative that, as far as can be
foreseen, any necessary change made to the MPLS data plane does not
impose any foreseeable future limitation on the MPLS data plane.
Stack size is an issue with many MPLS implementations both as a
result of hardware limitations, and due to the impact on networks and
applications where a large number of small payloads need to be
transported In particular one MPLS payload may be carried inside
another. For example, one LSP may be carried over another LSP, or a
PW or similar multiplexing construct may be carried over an LSP and
identification may be required at both layers. Of particular concern
is the implementation of low cost edge LSRs that for cost reasons
have a significant limit on the number of Label Stack Elements (LSEs)
that they can impose or dispose. Therefore, any method of identity
MUST NOT consume an excessive number of unique labels, and MUST NOT
result in an excessive increase in the size of the label stack.
The MPLS data plane design provides two types of special purpose
labels: the original 16 reserved labels and the much larger set of
special purpose labels defined in [RFC7274]. The original reserved
labels need one LSE, and the newer [RFC7274] special purpose labels
need two LSEs. Given the tiny number of original reserved labels, it
is core to the MPLS design philosophy that this scarce resource is
only used when it is absolutely necessary. Using a single LSE
reserved or special purpose label to encode flow identity thus
requires two stack entries, one for the reserved label and one for
the flow identity. The larger set of [RFC7274] labels requires two
labels stack entries for the special purpose label itself and hence a
total of three label stack entries to encode the flow identity.
The use of special purpose labels (SPL) [RFC7274] as part of a method
to encode the identity information therefore has a number of
undesirable implications for the data plane and hence whilst a
solution may use SPL(s), methods that do not require SPLs need to be
carefully considered.
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10. Control Plane
Any flow identity design should both seek to minimise the complexity
of the control plane and should minimise the amount of label co-
ordination needed amongst LSRs.
11. Privacy Considerations
The inclusion of originating and/or flow information in a packet
provides more identity information and hence potentially degrades the
privacy of the communication. Recent IETF concerns on pervasive
monitoring [RFC7258] would lead it to prefer a solution that does not
degrade the privacy of user traffic below that of an MPLS network not
implementing the flow identification feature. The choice of using
MPLS technology for this OAM solution has a privacy advantage as the
choice of the label identifying a flow is limited to the scope of the
MPLS domain and does not have any dependency on the user data's
identification. This minimizes the observability of the flow
characteristics.
12. Security Considerations
Any solution to the flow identification needs must not degrade the
security of the MPLS network below that of an equivalent network not
deploying the specified identity solution. Propagation of
identification information outside the MPLS network imposing it must
be disabled by default. Any solution should provide for the
restriction of the identity information to those components of the
network that need to know it. It is thus desirable to limit the
knowledge of the identify of an endpoint to only those LSRs that need
to participate in traffic flow. The choice of using MPLS technology
for this OAM solution, with MPLS encapsulation of user traffic,
provides for a key advantage over other data plane solutions, as it
provides for a controlled access and trusted domain within a Service
Provider's network.
For a more comprehensive discussion of MPLS security and attack
mitigation techniques, please see the Security Framework for MPLS and
GMPLS Networks [RFC5920].
13. IANA Considerations
This document has no IANA considerations. (At the discression of the
RFC Editor this section may be removed before publication).
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14. Acknowledgements
The authors thank Nobo Akiya, Nagendra Kumar Nainar, George Swallow
and Deborah Brungard for their comments.
15. References
15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, .
15.2. Informative References
[I-D.ietf-ippm-alt-mark]
Fioccola, G., Capello, A., Cociglio, M., Castaldelli, L.,
Chen, M., Zheng, L., Mirsky, G., and T. Mizrahi,
"Alternate Marking method for passive and hybrid
performance monitoring", draft-ietf-ippm-alt-mark-14 (work
in progress), December 2017.
[RFC5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
Label Assignment and Context-Specific Label Space",
RFC 5331, DOI 10.17487/RFC5331, August 2008,
.
[RFC5420] Farrel, A., Ed., Papadimitriou, D., Vasseur, JP., and A.
Ayyangarps, "Encoding of Attributes for MPLS LSP
Establishment Using Resource Reservation Protocol Traffic
Engineering (RSVP-TE)", RFC 5420, DOI 10.17487/RFC5420,
February 2009, .
[RFC5654] Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed.,
Sprecher, N., and S. Ueno, "Requirements of an MPLS
Transport Profile", RFC 5654, DOI 10.17487/RFC5654,
September 2009, .
[RFC5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
.
[RFC6374] Frost, D. and S. Bryant, "Packet Loss and Delay
Measurement for MPLS Networks", RFC 6374,
DOI 10.17487/RFC6374, September 2011, .
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[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, DOI 10.17487/RFC6790, November 2012,
.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, .
[RFC7274] Kompella, K., Andersson, L., and A. Farrel, "Allocating
and Retiring Special-Purpose MPLS Labels", RFC 7274,
DOI 10.17487/RFC7274, June 2014, .
Authors' Addresses
Stewart Bryant
Huawei
Email: stewart.bryant@gmail.com
Carlos Pignataro
Cisco Systems
Email: cpignata@cisco.com
Mach Chen
Huawei
Email: mach.chen@huawei.com
Zhenbin Li
Huawei
Email: lizhenbin@huawei.com
Gregory Mirsky
ZTE Corp.
Email: gregimirsky@gmail.com
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